Results of torsional ring shear, direct shear, and triaxial compression tests on cohesive soils reveal that the fully softened shear strength is stress - dependent and related to the type of clay mineral and quantity of clay - size particles. An empirical relationship for the fully softened friction angle is presented that is a function of liquid limit, clay size fraction, and effective normal stress. Studies of first time slides i.e. slopes that have not undergone previous sliding, in stiff fissured clay with  a liquid limit between 41 and 130% suggest that the mobilized shear strength along the failure surface can be lower than the fully softened shear strength. Recomendations are presented for estimating the mobilized shear strength in first time slides based on soil plasticity.